Methods for determining the charge state and/or the power capacity of a charge store

ABSTRACT

Methods are described for determining the state of charge and/or the operability of a charge accumulator using estimates, the information, which is obtained at at least two different operating points or operating conditions of the energy accumulator, being taken into account in the estimates. The estimates are carried out with regard to an instantaneous and/or future state of charge and/or an instantaneous and/or future operability of the charge accumulator. Different methods are executed, depending on the operating point or operating condition. The methods are usually run in a processor of a control unit.

BACKGROUND INFORMATION

[0001] Different methods for determining the state of charge andoperability of electric energy accumulators, in particular lead acidbatteries customary in the automotive industry, are known from therelated art. In most of the methods, the state of charge of lead acidbatteries is determined from the open-circuit voltage measured in theidling state, since the open-circuit voltage is proportional to the aciddensity in a broad range of states of charge (open-circuit voltagemethod). For the purpose of estimating the operability or load capacityof the energy accumulator with regard to a predetermined currentconsumption or power consumption, the internal resistance, which instarter batteries is ideally computed from the difference between themeasured voltage values divided by the difference between the measuredcurrent values during the high current load at engine start, is neededin addition to the open-circuit voltage or the state of charge. A methodused for determining the battery charge in that manner is known fromGerman Patent 198 47 648 for example.

[0002] Continuous information about the state of charge and theoperability of energy accumulators is required when safety-criticalelectrical consumers are used in motor vehicles, e.g., steer-by-wire orbrake-by-wire systems, but also for battery systems and consumermanagement systems, so that the open-circuit voltage and the state ofcharge must also be determined during charging and/or dischargingphases, and the internal resistance also without high current load. Forthis purpose, the state of charge is mostly extrapolated via the currentintegral using charge balancing and the internal resistance is mostlyextrapolated via fixed predefined characteristic curves as a function ofstate of charge and battery temperature. However, during extendedoperation of the energy accumulator without idle phases or high currentload, as well as due to the aging effects not taken into account in thecharacteristic curves, this method results in errors in the estimationof the state of charge and operability. To prevent these errors, therelated art describes model-based estimation methods which constantlyadjust the state variables and parameters of a mathematical model of theenergy accumulator to the real state variables and parameters bycontinuously measuring voltage, current, and temperature. Suchmodel-based estimation methods are known from German Patent 199 59 019.2for example. In the known methods, state of charge and operability ofthe energy accumulator are calculated from state variables andparameters so determined. The disadvantage of these methods is the factthat in order to cover the entire operating range of the energyaccumulator with regard to discharging-/charging current range, state ofcharge, temperature, as well as aging effects, a complex, and as a rulenon-linear, model of the energy accumulator is required, having manystate variables and parameters to be estimated and which may only beanalyzed at a great expense.

[0003] Alternatively simpler models covering only individual operatingpoints of the battery, e.g., only the discharging operation, haveadvantages; however, they allow an accurate determination of state ofcharge and operability only at these operating points. Such simplemodels are described in German Patent 100 56 969 for example.

OBJECT OF THE INVENTION

[0004] The object of the present invention is to make the most accuratedetermination of the state of charge and the operability of a chargeaccumulator possible over a large operating range without great expense.This object is achieved by using a method having the features ofclaim 1. By using a weighted correction of the state variables andparameters estimated from at least two methods that are active at twodifferent operating points via continuous measurement of voltage,current, and temperature, the method according to the present inventionmakes a more accurate estimation of the current and future state ofcharge and operability of the energy accumulator, in particular a motorvehicle lead battery, possible over a large operating range compared tothe individual methods.

ADVANTAGES OF THE INVENTION

[0005] The method according to the present invention having the featuresof claim 1 combines the advantage of the open-circuit voltage methods,i.e., the accurate determination of the open-circuit voltage, i.e., thestate of charge in phases of the battery without load and the internalresistance at high current load (e.g., engine start), and the advantageof simple model-based estimation methods using which open-circuitvoltage and internal resistance, as well as other optional statevariables and parameters may be estimated, even during operation withoutidling or high current loads, thereby, compared to the individualmethods, enabling a more accurate determination of the state of chargeand the operability of the battery over a large operating range withoutcomplex battery models.

[0006] Further advantages of the present invention are achieved by thefeatures stated in the subclaims. For calculating the state of chargeand the operability, the minimum required variables open-circuit voltageand internal resistance, as well as other optional state variables andparameters, are calculated in an advantageous manner from the values ofthe individual methods by weighted correction, their weighting beingselected according to their reliability at the current operating pointof the battery.

[0007] Predictions of the future operability are possible viaextrapolation of the currently estimated state variables and parametersfor state of charge and temperature to a later point in time, so that,for example, the capability of the battery to start a vehicle parked forseveral days may also be estimated.

DESCRIPTION

[0008]FIG. 1 shows the basic structure of the battery state detectionsystem using two state estimation and parameter estimation methodsactive at two different operating points of the battery. The number ofthe methods used is not necessarily limited to two; however, at leastone method is model-based, i.e., the state variables and parameters of abattery model are adapted to the real values, e.g., via a recursiveleast-square estimator, e.g., an extended Kalman filter.

[0009] State variables z (e.g., open-circuit voltage U₀₀) and parametersp (e.g., internal resistance R_(i)) required for determining the stateof charge and the operability of the battery are obtained fromcontinuous measurement of battery voltage U_(Batt), battery currentI_(Batt), and battery temperature T_(Batt), by the state and parameterestimating system. The state of charge calculation determines state ofcharge soc from vector z of the state variables and from theinstantaneous battery temperature T_(Batt), while the instantaneousoperability of the battery is estimated via voltage responseU_(Batt,pred)(t) of a battery model initialized using state vector z andparameter vector p to a predefined load current profile I_(Load)(t).

[0010] If the operability of the battery at a future point in time is ofinterest, e.g., the starting capability is queried after the vehicle wasparked for several days, then instantaneous variables z and parametersp, as well as instantaneous battery temperature T_(Batt) areextrapolated to values z′, p′, and T_(Batt)′ to be expected at thefuture point in time. In order to pre-estimate the reduction in thestate of charge as a function of the parking time of the vehicle, thevariation of closed-circuit current I_(Rest)(t) in the parked vehiclemust be known.

[0011] Using method A, here referred to as open-circuit voltage method,open-circuit voltage U₀₀ is determined during no-load phases of thebattery and internal resistance R_(i) of the battery is determinedduring a high current load (e.g., engine start). In addition, furthervariables, e.g., (acid) capacity Q₀, may be derived from measuredvariables current I_(Batt), voltage U_(Batt), and temperature T_(Batt),or calculated variables U₀₀ and R_(i). The state variables determined bymethod A are combined in state vector z_(A), and the parameters arecombined in vector p_(A). Method B is model-based and also estimates atleast open-circuit voltage U₀₀ and internal resistance R_(i), however,compared to method A, also in other or additional operating stages ofthe battery (e.g., discharge). The state variables determined by methodB are combined in state vector z_(B), and the parameters are combined invector p_(B).

[0012] In each calculation step k, state vector Z_(k+1) is calculated byusing weighted differences z_(A,k)−z_(k), z_(B,k)−z_(k) and parametervector p_(k+1) is calculated by using weighted differencesp_(A,k)−p_(k), p_(B,k)−p_(k) starting with starting values z₌₀=z₀ andp_(k−0)=p₀:

z _(k+1) =z _(k) +G _(z,A)*(z _(A,k) −z _(k))+G _(z,B)*(z _(B,k) −z_(k))

p _(k+1) =p _(k) +G _(p,A)*(p _(A,k) −p _(k))+G _(p,B)*(p _(B,k) =p_(k))

[0013] Weighting matrixes G_(z,A), G_(z,B), G_(p,A), and G_(p,B) aresquare diagonal matrixes whose main diagonal elementsg_(z,A,i=1 . . . n), g_(z,B,i=1 . . . n), g_(p,A,j=1 . . . m),g_(z,B,j=1 . . . m), specify the degree of correction of the n statevariables and the m parameters and must fulfil the followingrequirements, so that the sequences z_(k=0), Z_(k=1), z_(k=2) . . . andp_(k=0), p_(k=1), p_(k=2) . . . converge:

g _(z,A,i) +g _(z,B,i)≦1, i=1 . . . n

g _(p,A,j) +g _(p,B,j)≦1, j=1 . . . m

[0014] The weightings are selected in such a way that state variablesand parameters which at the instantaneous operating point are moreaccurately determined by using one method than the other, contributemore to the correction. For example, the estimated variables of themodel-based method may flow into the correction only when the estimatingalgorithm has become stable, when the estimated variables are uniquelyidentifiable (observability), and when the battery operates at pointswhich are also described by the underlying model (e.g., discharge). Inall other cases the corresponding weightings must be set g_(z,B,i) andg_(p,B,j)=0.

[0015] An example of a particular variant of an embodiment of thebattery state of charge detection for predicting the operability of leadbatteries in motor vehicles is described in the following:

[0016] Predictor Model

[0017] For estimating the operability of a lead battery under short-timeload (on the order of 10 sec) using currents on the order ofI_(Load)≦−100A (counting direction: I<0A for discharge) as it typicallyoccurs, e.g., in the operation of electric braking and steering systems,as well as at engine start in motor vehicles. The following simplepredictor model, illustrated in FIG. 2, may be used:

[0018] Using the equivalent diagram components:

[0019] I_(Load)=predefined load current for which operability is to betested

[0020] U₀₀=open-circuit voltage

[0021] R_(i)=ohmic internal resistance

[0022] Diode=non-linear resistance of the crossover polarization

[0023] U_(Ohm)=R_(i)*I_(Load)=ohmic voltage drop at predefined currentprofile I_(Load)

[0024] U_(D)=f(I_(Load), T_(batt))=characteristic curve of thestationary crossover voltage drop at predefined current profile I_(Load)and battery temperature T_(batt)

[0025] Formula determined from measurements:

U _(D)(I _(Load) ,U _(D0))=U _(D0) *ln(I _(Load)/(−1A)),I _(Load)<0A

[0026] using the temperature-dependent crossover parameter:

U _(D0)(T _(Batt))=4.97e−7*(T _(Batt) /° C.)³−4.87e−5*(T _(Batt) /°C.)²+1.82e−3*(T _(Batt) /° C.) . . . −1.31e−1

[0027] U_(Batt,pred)=U₀₀+R_(i)*I_(Load)+U_(D)(I_(Load),U_(D0))=predictedvoltage response for battery current I_(Load)

[0028] The following prerequisites must be met for the applicability ofthe predictor model:

[0029] the discharge due to the predefined load profile I_(Load)(t) isnegligible compared to the battery capacity, i.e., open-circuit voltageU₀₀ may be assumed to be constant,

[0030] during the load with I_(Load)(t), the crossover voltage becomesstabilized at its steady-state final value predefined by characteristiccurve U_(D)=f(I_(Load),T_(batt)), i.e., the load is applied sufficientlylong and is sufficiently high (time constant of U_(D)˜1/I_(Load)),

[0031] the concentration overvoltage, not considered in the model, whichis caused by acid density differences in the battery, is negligible,

[0032] charges which are possibly stored in additional capacitances(e.g., double layer capacitance between electrodes and electrolyte)outside the actual battery capacity are not considered (worst casescenario).

[0033] These prerequisites are met for the described load in the stateof charge range of soc>approximately 30% and for battery temperatures ofT_(Batt)>approximately 0° C., as well as soc>approximately 50% andT_(Batt)>approximately 0° C.

[0034] State variables and parameters are determined on the basis of thefollowing considerations:

[0035] State variable U₀₀, as well as parameters R_(i) and U_(D0) of thepredictor model, are determined by using two different methods:

[0036] Method A determines U_(00,A) from measurements of the idlingvoltage at unloaded battery and R_(i,A) by analyzing the quotient ofdifferences of the voltage and current values measured at engine start,while crossover parameter U_(D0,A) is not estimated by method A butrather calculated via the above-mentioned characteristic curve.

[0037] In addition, method A determines the battery (acid) capacity fromtwo open-circuit voltage determinations U_(00,A,1) and U_(00,A,2), aswell as the current integral (charge balance) q=∫I_(Batt)(t)dt:

Q _(0,A) =q*(U _(00,max)(25° C.)−U _(00,min)(25° C.))/(U _(00,A,2)(25°C.)−U _(00A,1)(25° C.))

[0038] where U_(00,max)=open-circuit voltage of the fully chargedbattery and U_(00,min)=open-circuit voltage of the empty battery atT_(Batt)=25° C.

[0039] Using Q_(0,A), current charge balance q_(k), and current batterytemperature T_(Batt,k), method A tracks open-circuit voltage U_(00,0),determined during the idle phase, during operation in each time step k:

U _(00,A,k)(25° C.)=U _(00,A,0)(25° C.)+q _(k) /Q _(0,A)*(U_(00,max)(25° C.)−U _(00,min)(25° C.)

U _(00,A,k) =U _(00,A,k)(25° C.)+Tk _(U00)*(T _(Batt,k)−25° C.), Tk_(U00)=1.38e−6V /° C.

[0040] Internal resistance R_(i,A,0), determined at the start, istracked in a similar manner during operation via a characteristic curveas a function of current open-circuit voltage U_(00,A,k) andinstantaneously measured battery temperature T_(Batt,k):

R _(i,k) =f(R _(i,A,0) , U _(00,A,k) , T _(Batt,k))

[0041] By adjusting a suitable battery model in discharge range(I_(Batt)<0A), method B estimates open-circuit voltage U_(00,B),internal resistance R_(i,B), as well as crossover parameter U_(D0,B),and battery capacity Q_(0,B). The variables needed for determining thestate of charge and operability are calculated from the state variablesand parameters determined by methods A and B using a weightedcorrection; a constant sampling rate of 0.01 sec has been assumed forthe individual time steps.

U _(00,k+1) =U _(00,k) +g _(U00,A)*(U _(00,A,k) −U _(00,k))+g_(U00,B)*(U _(00,B,k) −U _(00,k))

[0042] where U_(00,0)=U_(00,A,0), g_(U00,A)=1−|q_(k)|/Q₀,g_(U00,B)=|q_(k)|/Q₀

[0043] i.e., with an increasing absolute value of charge balance|q_(k)|, starting value U_(00,0)=U_(00,A,0) determined by method A froman idle phase is corrected to an increasing degree by value U_(00,B,k)determined by method B during vehicle operation.

R _(i,k+1) =R _(i,k) +g _(Ri,A)*(R _(i,A,k) −R _(i,k))+g _(Ri,B)*(R_(i,B,k) −R _(i,k))

[0044] where R_(i,0)=R_(i,A,0), g_(Ri,A)=0, g_(Ri,B)=1.e−3

[0045] i.e., starting value R_(i,0)=R_(i,A,0) determined by method A atengine start is corrected during vehicle operation to value R_(i,B,k)determined by method B using constant weighting g_(Ri,B)=1.e−3.

U _(D0,k+1) =U _(D0,k) +g _(UD0,A)*(U _(D0,A,k) −U _(D0,k))+g_(UD0,B)*(U _(D0,B,k) −U _(D0,k))

[0046] where U_(D0,0)=U_(D0,A,0), g_(UDO,A)=0, g_(UDO,B)=1.e−3

[0047] i.e., crossover parameter U_(D0,A) predefined by method A viacharacteristic curve U_(D0)(T_(Batt)) is corrected to value U_(D0,B,k)estimated by method B during vehicle operation using constant weightingg_(UD0,B)=1.e−3.

[0048] Capacity Q₀ is not really needed for the prediction ofoperability; however, value Q_(0,A,0) determined from idle phases bymethod A may be improved by values Q_(0,B,k) estimated by method Bduring vehicle operation. Since the accuracy of Q_(0,B,k) increases withincreasing absolute value of charge balance |q_(k)|, the weighting wasselected proportional to this value.

Q _(0,k+1) =Q _(0,k) +g _(Q0,A)*(Q _(0,A,k) −Q _(0,k))+g _(Q0,B)*(Q_(0,B,k) −Q _(0,k))

[0049] where Q_(0,0)=Q_(0,A,0), g_(Q0,A)=0,g_(Q0,B)=5.e−4*|q_(k)|/Q_(0,k)

[0050] Calculation of the Instantaneous State of Charge:

[0051] Relative state of charge soc is calculated from instantaneouslydetermined open-circuit voltage U₀₀ (state variable) and instantaneousbattery temperature T_(Batt) (measured variable):

soc=(U ₀₀(25° C.)−U _(00,min)(25° C.))/(U _(00,max)(25° C.)−U_(00,min)(25° C.)) where

[0052] U₀₀(25° C.)=U₀₀−Tk_(U00)*(T_(Batt)−25° C.), Tk_(U00)=1.38e−6V/°C.U_(00,max)(25° C.)=maximum value of the open-circuit voltage at roomtemperature and fully charged battery U_(00,min)(25° C.)=minimum valueof the open-circuit voltage at room temperature and empty battery (afterremoval of charge Q₀).

[0053] Calculation of the Instantaneous Operability

[0054] The instantaneous operability is determined by battery voltageU_(Batt,pred) under predefined load current I_(Load) calculated by usingthe predictor model, and the instantaneously estimated state variablesand parameters (U₀₀, R_(i), U_(D0)):

U _(Batt,pred) =U ₀₀ +R _(i) *I _(Load) +U _(D)(I _(Load) , U _(D0))

[0055] As the absolute measure for the operability of the energyaccumulator (SOH=State of Health), the distance of the minimum value ofthe predicted battery voltage to a lower limit voltage U_(Batt,limit) atwhich the energy accumulator just about generates the power required forthe considered user (e.g., electric steering and brake systems, starter,. . . ) may be used:

SOH _(abs)=min(U _(Batt,pred))−U_(Batt,limit)

[0056] The relative measure is obtained by relating SOH_(abs) to thedifference obtained in the most favorable case, i.e., for a new, fullycharged battery and at high temperatures:

SOH _(rel)=(minU_(batt,pred))−U _(batt,limit))/(U _(Batt,pred,max) −U_(batt,limit))

[0057] Calculation of Future Operability

[0058] Future operability may be estimated by inserting the statevariables (U_(oo)′) and parameters (R_(i)′, U_(D0)′), extrapolated tothe future point in time with regard to battery temperature and state ofcharge, into the prediction equation. Temperature T_(Batt)′ to beexpected may be determined by averaging the battery temperatures overthe previous 10 to 14 days. For worst case scenarios, 10K are once moresubtracted from this value.

[0059] Open-circuit voltage U₀₀′ to be expected after x days of parkingof the vehicle is determined via the drop in the state of charge basedon the discharge due to closed-circuit current I_(Rest):

U ₀₀(25° C.)′=U ₀₀(25° C.)+I _(Rest) *x*24 h/Q ₀*(U _(00,max)(25° C.)−U_(00,min)(25° C.))

U ₀₀ ′=U ₀₀(25° C.)′+Tk _(U00)*(T _(Batt)′−25° C.), Tk _(U00)=1.38e−6V°C.

[0060] Internal resistance R_(i)′ is extrapolated by usingcharacteristic curve R_(i)′=f(R_(i), U₀₀′, T_(Batt)′), while crossoverparameter U_(D0)′ is calculated via characteristic curveU_(D0)(T_(Batt)′).

What is claimed is:
 1. A method for determining the state of chargeand/or the operability of a charge accumulator using estimates, whereininformation obtained at at least two different operating points of theenergy accumulator is taken into account in the estimates.
 2. The methodfor determining the state of charge and/or the operability of a chargeaccumulator as recited in claim 1, wherein at least two differentmethods are executed, in particular an open-circuit voltage method and amodel-based estimation method, and the information obtained in each caseis taken into account for determining the state of charge.
 3. The methodfor determining the state of charge and/or the operability of a chargeaccumulator as recited in claim 1, wherein weightable correctionvariables are formed from the information.
 4. The method for determiningthe state of charge and/or the operability of a charge accumulator asrecited in claim 1, 2, or 3, wherein the estimations are carried outwith regard to an instantaneous and/or future state of charge, and/or aninstantaneous and/or a future operability of the charge accumulator. 5.The method for determining the state of charge and/or the operability ofa charge accumulator as recited in claim 3, wherein the estimation withregard to the future state of charge and/or the future operability ofthe charge accumulator are carried out by using a predictor.
 6. Themethod for determining the state of charge and/or the operability of acharge accumulator as recited in one of the preceding claims, whereinthe two operating states are the idle state of the charge accumulatorand an active state of the charge accumulator.
 7. The method fordetermining the state of charge and/or the operability of a chargeaccumulator as recited in one of the preceding claims, wherein a stateestimate and a parameter estimate are implemented.
 8. The method fordetermining the state of charge and/or the operability of a chargeaccumulator as recited in claim 6, wherein an open-circuit voltagemethod and a model-based estimation method are used for the stateestimate and the parameter estimate.
 9. The method for determining thestate of charge and/or the operability of a charge accumulator asrecited in one of the preceding claims, wherein mathematical models areformed which are processable in a processor taking predefinablevariables into account.
 10. A method for determining the state of chargeand/or the operability of a charge accumulator, wherein state variablesand parameters are determined from the following measured variables:battery voltage, battery current, and battery temperature according to afirst method A, in particular an open-circuit voltage method; andadditional state variables and parameters are determined according to asecond, model-based method B and correction variables are obtainedtherefrom; and the state variables are used for calculating the state ofcharge, and the state variables and the parameters are used forpredicting the operability of the charge accumulator and for determiningthe predetermined battery voltage.
 11. A device for carrying out atleast one method of determining the state of charge and/or theoperability of a charge accumulator as recited in one of the precedingclaims, wherein the device includes at least one processor, which is inparticular a component of a control unit, to which the requiredinformation is supplied.
 12. The device for determining the state ofcharge and/or the operability of a charge accumulator as recited inclaim 11, wherein it has means for estimating the state and parameters,means for calculating the state of charge, and means for predicting theoperability, which are connected to one another.
 13. (New) A method fordetermining at least one of a state of charge and an operability of acharge accumulator in accordance with an estimate, comprising: takinginto account in the estimate information obtained at at least twodifferent operating points of the charge accumulator.
 14. (New) Themethod as recited in claim 13, further comprising: executing at least anopen-circuit voltage operation and a model-based estimation operation;and determining the state of charge in accordance with informationobtained in each of the open-circuit voltage operation and themodel-based estimation operation.
 15. (New) The method as recited inclaim 14, further comprising: forming a weightable correction variablefrom the information.
 16. (New) The method as recited in claim 13,further comprising: performing an estimation in accordance with at leastone of: at least one of an instantaneous state of charge and a futurestate of charge, and at least one of an instantaneous operability of thecharge accumulator and a future operability of the charge accumulator.17. (New) The method as recited in claim 15, further comprising:operating a predictor to perform an estimation in accordance with atleast one of a future state of charge of the charge accumulator and afuture operability of the charge accumulator.
 18. (New) The method asrecited in claim 13, wherein: two operating states include an idle stateof the charge accumulator and an active state of the charge accumulator.19. (New) The method as recited in claim 13, further comprising:implementing a state estimate and a parameter estimate.
 20. (New) Themethod as recited in claim 19, wherein: an open-circuit voltageoperation and a model-based estimation operation are used for the stateestimate and the parameter estimate.
 21. (New) The method as recited inclaim 13, further comprising: forming a mathematical model that isprocessable in a processor in accordance with a predefinable variable.22. (New) A method for determining at least one of a state of charge andan operability of a charge accumulator in accordance with an estimate,comprising: determining a state variable and a state parameter from thefollowing measured variables: a battery voltage, a battery current, anda battery temperature according to a first operation; determining anadditional state variable and an additional sate parameter according toa second, model-based operation; obtaining a correction variable from atleast one of the state variable, the additional state variable, thestate parameter, and the additional state parameter; calculating thestate of charge in accordance with at least one of the state variableand the additional state variable; and predicting the operability of thecharge accumulator and determining the battery voltage in accordancewith at least one of the state variable and the additional statevariable and at least one of the state parameter and the additionalstate parameter.
 23. (New) The method as recited in claim 22, wherein:the first operation includes an open-circuit voltage operation. 24.(New) A device for determining at least one of a state of charge and anoperability of a charge accumulator in accordance with an estimate,comprising: an arrangement for taking into account in the estimateinformation obtained at at least two different operating points of thecharge accumulator; and a control unit including at least one processorand to which required information is supplied.